typo: Use plural even if we only have a single list item

Since we speak of *all* extensions (even if we only have one in the
document), we need to use the plural form of the word extension.

Signed-off-by: Christoph Müllner <[email protected]>

Author: cmuellner

README.mkd

@@ -246,7 +246,7 @@ As of November 2021 the additional tail-call entry points are only
 implemented in compiler-rt, and calls will only be generated by LLVM
 when the option `-mllvm -save-restore-tailcall` is specified.
 
-## Conventions for vendor extension
+## Conventions for vendor extensions
 
 Support for custom instruction set extensions are an important part of RISC-V,
 with large encoding spaces reserved of vendor extensions.
@@ -301,7 +301,7 @@ Ventana Micro Systems  | vt              | https://www.ventanamicro.com/
 
 NOTE: Vendor prefixes are case-insensitive.
 
-### List of vendor extension
+### List of vendor extensions
 
 Vendor  | Name            | Version        | ISA Document
 :------ | :-------------- | :------------- | :---------------