Add tag to senvcfg section (riscv#1562)

src/supervisor.adoc

@@ -711,6 +711,7 @@ instruction bits is implemented, `stval` must also be able to hold all
 values less than latexmath:[$2^N$], where latexmath:[$N$] is the smaller
 of SXLEN and ILEN.
 
+[[sec:senvcfg]]
 ==== Supervisor Environment Configuration (`senvcfg`) Register 
 
 The `senvcfg` CSR is an SXLEN-bit read/write register, formatted as