Replace "Sv39" with "Sv39 extension" where appropriate (riscv#1913)

Author: aswaterman

src/supervisor.adoc

@@ -1863,7 +1863,7 @@ translations with the same values for PTE bits 5–0. Such ranges must be
 of a naturally aligned power-of-2 (NAPOT) granularity larger than the
 base page size.
 
-The Svnapot extension depends on Sv39.
+The Svnapot extension depends on the Sv39 extension.
 
 [[ptenapot]]
 .Page table entry encodings when __pte__.N=1
@@ -2028,7 +2028,7 @@ the use of page-based memory types that override the PMA(s) for the
 associated memory pages. The encoding for the PBMT bits is captured in
 <<pbmt>>.
 
-The Svpbmt extension depends on Sv39.
+The Svpbmt extension depends on the Sv39 extension.
 
 [[pbmt]]
 .Encodings for PBMT field in Sv39, Sv48, and Sv57 PTEs.