qemu-doc.texi: Fix capitalization error in OS X build instructions

Fix a capitalization error in the OS X build instructions;
this was picked up in review of commit b352153f and intended to be
corrected before I applied it, but I accidentally didn't include it.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

qemu-doc.texi

@@ -3062,7 +3062,7 @@ Note: If after the configure step you see a message like this:
 ERROR: Your compiler does not support the __thread specifier for
        Thread-Local Storage (TLS). Please upgrade to a version that does.
 @end example
-You may have to build your own version of gcc from source. Expect that to take
+you may have to build your own version of gcc from source. Expect that to take
 several hours. More information can be found here:
 @uref{https://gcc.gnu.org/install/} @*