dev-manual: correct statements that source is always tarballs

Correct two occurrences that imply source is always fetched as
tarballs.

(From yocto-docs rev: 57490852e6446672fd2cd0f3ee0befd9714c909f)

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/dev-manual/efficiently-fetching-sources.rst

@@ -6,7 +6,7 @@ Efficiently Fetching Source Files During a Build
 The OpenEmbedded build system works with source files located through
 the :term:`SRC_URI` variable. When
 you build something using BitBake, a big part of the operation is
-locating and downloading all the source tarballs. For images,
+locating and downloading all of the source code. For images,
 downloading all the source for various packages can take a significant
 amount of time.
 
@@ -18,7 +18,7 @@ Setting up Effective Mirrors
 ============================
 
 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
+source code. Maybe you have been working with another
 build system for which you have built up a
 sizable directory of source tarballs. Or, perhaps someone else has such
 a directory for which you have read access. If so, you can save time by