Documentation: GNU is frequently spelled Gnu

The official spelling of GNU is GNU and not Gnu.
Bug 89551 https://bugzilla.kernel.org/show_bug.cgi?id=89551

Signed-off-by: Kevin Law <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/Changes

···
 systems; obviously, if you don't have any ISDN hardware, for example,
 you probably needn't concern yourself with isdn4k-utils.
 
-o  Gnu C                  3.2                     # gcc --version
-o  Gnu make               3.80                    # make --version
+o  GNU C                  3.2                     # gcc --version
+o  GNU make               3.80                    # make --version
 o  binutils               2.12                    # ld -v
 o  util-linux             2.10o                   # fdformat --version
 o  module-init-tools      0.9.10                  # depmod -V
···
 Make
 ----
 
-You will need Gnu make 3.80 or later to build the kernel.
+You will need GNU make 3.80 or later to build the kernel.
 
 Binutils
 --------