doc/build/gcc.rst: add missing apt-get *install*

The install command is missing for an apt-get command.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/build/gcc.rst

@@ -17,7 +17,7 @@ You could install GCC and the GCC cross compiler for the ARMv8 architecture with
 
 .. code-block:: bash
 
-    sudo apt-get gcc gcc-aarch64-linux-gnu
+    sudo apt-get install gcc gcc-aarch64-linux-gnu
 
 Depending on the build targets further packages maybe needed