Typo fix in intro.tex

Fixes small typo "ans earth impacting asteroid" -> " an Earth-impacting asteroid"

intro.tex

@@ -42,7 +42,7 @@ \section{Introduction}
 Additionally no prompt alerts will be issued for sources corresponding to objects in an appropriate satellite catalog.
 And no prompt alerts for streaks greater than 10 \gls{deg}/day relative to sidereal.
 The exclusion of prompt alerts for these streaks will have minimal impact on science.
-In the rare event an unknown solar system object source with a streak length greater than 10 \gls{deg}/day relative to sidereal potentially corresponds with ans earth impacting asteroid, the \gls{USDF} will implement a method to send any candidate impactors to the \gls{MPC} during the embargo period.
+In the rare event an unknown solar system object source with a streak length greater than 10 \gls{deg}/day relative to sidereal potentially corresponds with an Earth-impacting asteroid, the \gls{USDF} will implement a method to send any candidate impactors to the \gls{MPC} during the embargo period.
 All data in transit shall comply with the strong, approved encryption standards outlined for the embargo period.
 
 In addition, commissioning and engineering data will be embargoed for all non-commissioning team staff for 30 days.