fix: change "add new slice" page title copy to "add new chart"

[riahk]

SUMMARY

Pretty much what the title says. Updates page title for adding a chart to better match its content.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before

After

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

[codecov-commenter]

Codecov Report

Merging #10457 into master will increase coverage by 6.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10457      +/-   ##
==========================================
+ Coverage   64.29%   70.37%   +6.08%     
==========================================
  Files         544      604      +60     
  Lines       30586    32381    +1795     
  Branches     2885     3272     +387     
==========================================
+ Hits        19665    22788    +3123     
+ Misses      10743     9483    -1260     
+ Partials      178      110      -68     

Flag	Coverage Δ
#cypress	54.80% <ø> (-0.31%)	⬇️
#javascript	59.16% <ø> (?)
#python	69.83% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset-frontend/src/setup/setupPlugins.ts	22.22% <0.00%> (-77.78%)	⬇️
superset-frontend/src/SqlLab/index.tsx	25.00% <0.00%> (-75.00%)	⬇️
...rset-frontend/src/setup/setupErrorMessagesExtra.ts	50.00% <0.00%> (-50.00%)	⬇️
superset-frontend/src/setup/setupErrorMessages.ts	55.55% <0.00%> (-44.45%)	⬇️
superset-frontend/src/preamble.ts	56.52% <0.00%> (-24.73%)	⬇️
...s/ErrorMessage/getErrorMessageComponentRegistry.ts	85.71% <0.00%> (-14.29%)	⬇️
...et-frontend/src/components/RefreshChartOverlay.tsx	90.00% <0.00%> (-10.00%)	⬇️
.../src/components/Select/WindowedSelect/windowed.tsx	94.11% <0.00%> (-5.89%)	⬇️
superset-frontend/src/setup/setupApp.ts	25.00% <0.00%> (-3.58%)	⬇️
...rc/components/ErrorMessage/TimeoutErrorMessage.tsx	5.45% <0.00%> (-1.22%)	⬇️
... and 221 more

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