Avoid "should" when describing a mandate

Resolves riscv#510

src/machine.tex

@@ -95,8 +95,8 @@ \subsection{Machine ISA Register {\tt misa}}
 RV32I, RV64I, RV128I base ISAs, and the ``E'' bit will be set for
 RV32E.  The Extensions field is a \warl\ field that can contain writable bits
 where the implementation allows the supported ISA to be modified.  At
-reset, the Extensions field should contain the maximal set of supported
-extensions, and I should be selected over E if both are available.
+reset, the Extensions field shall contain the maximal set of supported
+extensions, and I shall be selected over E if both are available.
 
 The RV128I base ISA is not yet frozen, and while much of the remainder of this
 specification is expected to apply to RV128, this version of the document