doc: build: fix typo in gcc.rst

Fix a typo error in gcc.rst, changing "out-out-tree" to
"out-of-tree" in the Out-of-tree section.

Signed-off-by: BehradElmi <behradelmi1@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index c76a7fb..1fef718 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -122,7 +122,7 @@
 ~~~~~~~~~~~~~~~~~~~~
 
 By default building is performed locally and the objects are saved in the source
-directory. To build out-out-tree use one of the two methods below:
+directory. To build out-of-tree use one of the two methods below:
 
 Add O= parameter to the make command line: