Doc: simplify \score description, matching its current syntax (issue 47900043)

[tdanielsmusic]

One suggested improvement, otherwise LGTM

Trevor



https://codereview.appspot.com/47900043/diff/1/Documentation/learning/fundamental.itely
File Documentation/learning/fundamental.itely (right):

https://codereview.appspot.com/47900043/diff/1/Documentation/learning/fundamental.itely#newcode271
Documentation/learning/fundamental.itely:271: @emph{A @code{\score}
block must contain a music expression.}
Perhaps to avoid ambiguity,
"must contain exactly one music expression" (as before) or
"must contain a single music expression" (as a variation).

https://codereview.appspot.com/47900043/