Fix a typo in mm-formal.adoc (riscv#1901)

Author: pfusik

src/mm-formal.adoc

@@ -735,7 +735,7 @@ register write by the most recent (in program order) instruction
 instance that can write that bit (or from the hart’s initial register
 state if there is no such write). Hence, it is essential to know the
 register write footprint of each instruction instance, which we
-calculate when the instruction instance is created (see the <<fetch, Festch instruction>> action of
+calculate when the instruction instance is created (see the <<fetch, Fetch instruction>> action of
 below). We ensure in the pseudocode that each instruction does at most
 one register write to each register bit, and also that it does not try
 to read a register value it just wrote.