fix(expansion): use correct expanded line-height

[willshowell]

Presumed typo in animation styles. It now matches the value from the component styles

$mat-expansion-panel-header-height-expanded: 64px;

...

&.mat-expanded {
  height: $mat-expansion-panel-header-height-expanded;
  line-height: $mat-expansion-panel-header-height-expanded;
}

[crisbeto]

LGTM, CC @josephperrott

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