dev-manual: Changed heading for mirror and pre-fectch section

The old heading was dangerously close to a heading in the
previous chapter.  I altered this heading to target more
specifically the content.

(From yocto-docs rev: 797bef2256308531f02e35dc0ef8b1d9cbaff935)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/dev-manual-common-tasks.xml

@@ -8977,8 +8977,8 @@ Some notes from Cal:
         </section>
     </section>
 
-    <section id='working-with-source-files'>
+    <section id='efficiently-fetching-source-files-during-a-build'>
-        <title>Working with Source Files</title>
+        <title>Efficiently Fetching Source Files During a Build</title>
 
         <para>
             The OpenEmbedded build system works with source files located
@@ -8992,16 +8992,16 @@ Some notes from Cal:
         </para>
 
         <para>
-            This section presents information for working with source
+            This section shows you how you can use mirrors to speed up
-            files that can lead to more efficient use of resources and
+            fetching source files and how you can pre-fetch files all of which
-            time.
+            leads to more efficient use of resources and time.
         </para>
 
         <section id='setting-up-effective-mirrors'>
             <title>Setting up Effective Mirrors</title>
 
             <para>
-                As mentioned, a good deal that goes into a Yocto Project
+                A good deal that goes into a Yocto Project
                 build is simply downloading all of the source tarballs.
                 Maybe you have been working with another build system
                 (OpenEmbedded or Angstrom) for which you have built up a