Doc: Use \c instead of \tt for literal text in the Linux guide

The Linux platform pages mark up configure options such as -xrender
with \tt, while the same pages use \c for other configure options.
The two commands produce the same output; \tt only differs in that it
lets other QDoc commands be nested inside its argument, and none of
these options need that.

Replace \tt with \c where the argument is plain text. Keep \tt where
the argument contains the \QtVersion macro, because \c takes its
argument verbatim and would print the macro name instead of the
version.

Pick-to: 6.12
Change-Id: Ifcdedb4140b9d17f78b7ca6413572e8a5799035a
Reviewed-by: Topi Reinio <topi.reinio@qt.io>