disas: nanoMIPS: Fix an FP-related misnomer 2

Rename NMD::extract_fs_15_14_13_12_11(uint64 instruction) to
NMD::extract_fs_20_19_18_17_16(uint64 instruction).
Reviewed-by: NAleksandar Rikalo <arikalo@wavecomp.com>
Signed-off-by: NAleksandar Markovic <amarkovic@wavecomp.com>

disas/nanomips.h

@@ -177,7 +177,7 @@ private:
     uint64 extract_eu_3_2_1_0(uint64 instruction);
     uint64 extract_eu_6_5_4_3_2_1_0(uint64 instruction);
     uint64 extract_fd_15_14_13_12_11(uint64 instruction);
-    uint64 extract_fs_15_14_13_12_11(uint64 instruction);
+    uint64 extract_fs_20_19_18_17_16(uint64 instruction);
     uint64 extract_ft_15_14_13_12_11(uint64 instruction);
     uint64 extract_ft_20_19_18_17_16(uint64 instruction);
     uint64 extract_gp_2(uint64 instruction);