[doc] install.texi -- update link for MPC

[Gerald Pfeifer]

Committed.  

Probably not worth backporting to the GCC 8, 7, and 6 branches?

Gerald

2018-09-01  Gerald Pfeifer  <gerald@pfeifer.com>

	* doc/install.texi (Prerequisites): Update link for MPC.

Index: doc/install.texi
===================================================================
--- doc/install.texi	(revision 264032)
+++ doc/install.texi	(working copy)
@@ -378,7 +378,7 @@
 @item MPC Library version 0.8.1 (or later)
 
 Necessary to build GCC@.  It can be downloaded from
-@uref{http://www.multiprecision.org/}.  If an MPC source distribution
+@uref{http://www.multiprecision.org/mpc/}.  If an MPC source distribution
 is found in a subdirectory of your GCC sources named @file{mpc}, it
 will be built together with GCC.  Alternatively, if MPC is already
 installed but it is not in your default library search path, the