Minor name change update for plots tests (#4532)

Just a name change for a button to fix tests

### QA Notes

All smoke tests should pass.

test/automation/src/positron/positronPlots.ts

@@ -16,7 +16,7 @@ const CLEAR_PLOTS_BUTTON = '.positron-plots-container .positron-action-bar .posi
 const PLOT_SIZE_BUTTON = '.positron-plots-container .positron-action-bar .positron-button[aria-label="Auto"]';
 const SAVE_PLOT_BUTTON = '.positron-plots-container .positron-action-bar .positron-button[aria-label="Save plot"]';
 const COPY_PLOT_BUTTON = '.positron-plots-container .positron-action-bar .positron-button[aria-label="Copy plot to clipboard"]';
-const ZOOM_PLOT_BUTTON = '.positron-plots-container .positron-action-bar .positron-button[aria-label="Fill"]';
+const ZOOM_PLOT_BUTTON = '.positron-plots-container .positron-action-bar .positron-button[aria-label="Fit"]';
 const OUTER_WEBVIEW_FRAME = '.webview';
 const INNER_WEBVIEW_FRAME = '#active-frame';