Fix the title of m-st-ext.adoc

Modify 'M' to "M".

Signed-off-by: kkqq <[email protected]>

Author: zhangkanqi

src/m-st-ext.adoc

@@ -1,5 +1,5 @@
 [[mstandard]]
-== `M` Extension for Integer Multiplication and Division, Version 2.0
+== "M" Extension for Integer Multiplication and Division, Version 2.0
 
 This chapter describes the standard integer multiplication and division
 instruction extension, which is named `M` and contains instructions