Docs: Fix broken header references

MyST Parser automatically generates label "slugs" for headers which
should be used to reference them from links [1]. These labels are in
"slug-case", i.e. the original header text in lower case separated by
dashes, with punctuation removed. This fixes a few "cross-reference
target not found" warnings.

[1] https://myst-parser.readthedocs.io/en/latest/syntax/optional.html#anchor-slug-structure

Change-Id: Ia6970d03b961bde6d7cd0fa3297f8d84b75d3b34
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83976
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/northbridge/intel/sandybridge/nri_freq.md

@@ -160,7 +160,7 @@ As you can see, by using DIMMs with different maximum DRAM frequencies, the
 slowest DIMMs' frequency will be selected, to prevent over-clocking it.
 
 The selected frequency gives the PLL multiplier to operate at. In case the PLL
-locks (see Take me to [Hard fuses](#hard_fuses)) the frequency will be used for
+locks (see Take me to [Hard fuses](#hard-fuses)) the frequency will be used for
 all DIMMs. At this point it's not possible to change the multiplier again,
 until the system has been powered off. In case the PLL doesn't lock, the next
 smaller multiplier will be used until a working multiplier will be found.