[doc] fix small mistakes in engine overview

_sources/dev/engine_overview.txt

 (normally ``settings.yml``). The arguments in the settings file override
 the ones in the engine file.
 
-It does not matter if an options is stored in the engine file or in the
+It does not matter if an option is stored in the engine file or in the
 settings. However, the standard way is the following:
 
 
 in the engine file, it has to be redefined in the settings,
 otherwise searx will not start with that engine.
 
-The naming of that overrides is arbitrary. But the recommended
+The naming of overrides is arbitrary. But the recommended
 overrides are the following:
 
 +-----------------------+----------+----------------------------------------------------------------+

dev/engine_overview.html

 arguments can be set in the engine file or in the settings file
 (normally <code class="docutils literal"><span class="pre">settings.yml</span></code>). The arguments in the settings file override
 the ones in the engine file.</p>
-<p>It does not matter if an options is stored in the engine file or in the
+<p>It does not matter if an option is stored in the engine file or in the
 settings. However, the standard way is the following:</p>
 <div class="section" id="engine-file">
 <h3><a class="toc-backref" href="#id4">engine file</a><a class="headerlink" href="#engine-file" title="Permalink to this headline">¶</a></h3>
 often overwritten by the settings. If <code class="docutils literal"><span class="pre">None</span></code> is assigned to an option
 in the engine file, it has to be redefined in the settings,
 otherwise searx will not start with that engine.</p>
-<p>The naming of that overrides is arbitrary. But the recommended
+<p>The naming of overrides is arbitrary. But the recommended
 overrides are the following:</p>
 <table border="1" class="docutils">
 <colgroup>

docs/dev/engine_overview.rst

 (normally ``settings.yml``). The arguments in the settings file override
 the ones in the engine file.
 
-It does not matter if an options is stored in the engine file or in the
+It does not matter if an option is stored in the engine file or in the
 settings. However, the standard way is the following:
 
 
 in the engine file, it has to be redefined in the settings,
 otherwise searx will not start with that engine.
 
-The naming of that overrides is arbitrary. But the recommended
+The naming of overrides is arbitrary. But the recommended
 overrides are the following:
 
 +-----------------------+----------+----------------------------------------------------------------+