Fix unconventional spelling of ñg to n¯g in Yale

[AlienKevin]

On the one hand, the syllable m1 is spelled as m¯ (m with macron) in Yale, which follows the convention in using the macron as the mark for tone 1. On the other hand, ng1 is spelled as ñg in Yale, which may be a convenient replacement for n¯ (n with macron) that is not a single character in Unicode. However, it can confuse readers not familiar with this tilde macron shorthand. Thus, I suggest we stick to the conventional notation and use n¯ instead.

Edit: I also noticed an issue with the encoding of the macrons. It is currently encoded as a separate glyph U+00AF MACRON but I think it should be U+0304 COMBINING MACRON instead. So m¯ should be m̄, and n¯g should be n̄g instead. The font used by Github shifts the combining macron to the right but in fonts that support the symbol, the macron should be right on top of the letter.