[flang] Update docs link to latest PRIF Specification (llvm#159233)

The PRIF Committee is pleased to announce the publication of the
Parallel
Runtime Interface for Fortran (PRIF) Specification, Revision 0.6. The
latest
iteration of this specification represents the efforts of a
collaborative
design process involving multiple individuals across several
institutions.

The document is available here: <https://doi.org/10.25344/S4M01X>

The PRIF specification is now governed by a formal PRIF Committee.
For more details, see: <https://go.lbl.gov/prif-governance>

The Committee vote to approve the technical content in this revision
began on 2025-08-22 and concluded on 2025-09-07 with unanimous approval.

The 7-day Committee comment period for cosmetic feedback began on
2025-09-08 and concluded on 2025-09-15 with no comments.

See the Change Log in Section 1 of the document for the list of changes
relative to the prior revision.

Author: bonachea

flang/docs/ParallelMultiImageFortranRuntime.md

@@ -14,5 +14,5 @@ interface designed for LLVM Flang to target implementations of
 Fortran's multi-image parallel features.
 
 The current revision of the PRIF specification is here:
-<https://doi.org/10.25344/S4CG6G>
+<https://doi.org/10.25344/S4M01X>